1. Field of the Invention
The present invention relates to an implantable medical device including a rechargeable back-up power source and a charging unit for recharging the back-up power source via RF coupling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR §§1.97-1.99
The concept of using an implantable, electrically operated medical device for treating specific diseases or physical disorders is well known. Examples of implantable, electrically operated medical devices are: cardiac pacemakers which restore a sick human heart to a normal rhythm, neural simulators which control nerve or brain response (such as pain or epileptic seizures), infusion pumps for subcutaneously drug delivery (such as insulin pump), and diagnostic devices for monitoring a patient's condition.
With respect to all of these implantable, electrically operated devices, it is necessary to provide power to the device implanted below the skin. Since the medical device is subcutaneously implanted in the patient, the power source must supply electrical energy for a reasonable period of time in order to reduce further surgical trauma to the patient and financial cost to the medical provider.
Implantable, electrically operated medical devices can be classified in three general areas: radio frequency (RF) coupled and partially powered implanted devices, non-rechargeable battery powered totally implanted devices, and devices which combine RF coupling and battery powered systems.
The RF coupled and powered devices do not carry or contain an independent power source. Therefore, the RF coupled device requires an external RF transmitter and a surgically implanted receiver. Such a device is an RF coupled neural stimulator. The RF link transfers stimulation pulses percutaneously through the skin and adjacent tissue layers of the patient from the external RF transmitter to the surgically implanted RF receiver and stimulator device. The transmitter sends, on a real-time basis, stimulation pulses to be applied ultimately to the implanted electrodes plus programming data defining the polarity of each electrode relative to each other to the implanted stimulation device. The implanted receiver obtains these stimulation pulses and programming data, converts pulses as necessary, delivers the energy contained in each transmitted stimulation pulse to the implanted electrodes as defined in the real-time programming data. The stimulation energy for each stimulation pulse is inductively coupled on a real-time basis from the external transmitter to the implanted receiver.
The common disadvantage of the RF coupled and powered stimulator is that the patient must always wear the external transmitter and antenna (even during sleep) in order for the implanted receiver to deliver stimulation pulses to the targeted tissue. Stimulation therapy ceases the moment the transmitter antenna is withdrawn just a few inches away from the implanted receiver. Although the RF coupled stimulator has this disadvantage, the service life of such an RF coupled and powered stimulator is not limited to the life of a battery contained within a fully implantable stimulation unit. Accordingly, the long term cost of the RF coupled and powered simulators is less than the non-rechargeable battery powered simulators because the service life of the former is much longer than that of the latter. RF coupled and powered simulators have been commercially marketed by Medtronic of Minneapolis, Avery Laboratories of New York and Neuromed of Dallas, Tex.
The battery powered stimulator utilizes a primary, non-rechargeable battery to power the implanted stimulator. The battery provides sole and exclusive power to the implanted stimulator continually while the stimulator generates one or more electric stimulation pulses, in a controlled manner, to the targeted tissue. Of course, the stimulation pulses are delivered to the targeted tissue via implanted leads. An external programmer may be used to non-invasively adjust the stimulation parameters, such as amplitude, pulse width or rate, or to control the duration of stimulation therapy each day. Programming may be provided through an RF telemetry link. After programming, the stimulator remembers the parameter values (the values are stored in an electronic memory) as long as the battery voltage remains above a minimum voltage level required by the electronics. Unfortunately, the service life of these battery powered implantable simulators is limited to the battery life. Accordingly, it is necessary to surgically remove and then replace the battery powered implantable simulators upon depletion of the electrochemically active materials in the battery. This disadvantage (i.e. surgical replacement) increases its long term cost to the medical provider relative to the aforementioned RF coupled and powered simulators. The battery powered implantable simulators do not require an external transmitter to deliver the stimulating electrical pulses. Accordingly, the battery powered implantable simulators are less complicated to use and more comfortable to wear than the RF coupled and powered simulators. Battery powered simulators have been marketed by Medtronic of Minneapolis and Exonix of Miami.
A third category of implantable, electrically operated devices include neural simulators which combine the RF coupled/powered systems with the battery powered implantable stimulator technology. These types of simulators enable the patient to obtain therapy without the necessity of having an external RF coupled unit proximate to the implant at all times. However, the stimulator must be surgically replaced after the battery is depleted if use of the external RF transmitter is not desired. This type of stimulator allows RF coupled stimulation at times when wearing the external transmitter is not objectionable, thereby extending battery life. Also, this type of stimulator may allow for continuing RF coupled stimulation after the internal rower source is depleted, although some of these RF coupled and battery powered implantable simulators do not operate if the battery is completely depleted in the implanted stimulator.
Several examples of such previously proposed implantable devices are disclosed in the following U.S. patents:
U.S. Pat. No.Patentee4,408,607Maurer4,793,353Borkan5,279,292Baumann et al.5,314,453Jeutter
U.S. Pat. No. 5,314,453 to Jeutter describes and claims an implant “locator” means to aid in locating an implanted device. The locator means comprises a reed switch affixed to the center of a transmitting coil and a magnet affixed within the implanted device. Transmission of high frequency RF energy is possible only when the reed switch is closed by the magnet within the implanted device, thus insuring some degree of good coupling between the transmitter and receiver. However, this patent describes a full wave rectifier along with rechargeable batteries. Accordingly, it incorporates a rechargeable battery. The present invention differs from the teachings of the Jeutter patent in the following respects:                1) Charging current is controlled by battery temperature to prevent gas generation by the battery and loss of battery electrolyte,        2) A real-time feedback system is provided between the receiver and the transmitter for real-time adjustment of the RF energy generated by the transmitter, thereby extending the service time of the transmitter's battery.        3) A low frequency RF coupling method (10 to 500,000 Hertz) is provided which allows RF coupling through a titanium encased receiver. The RF coupling system described by Jeutter operates at a very high frequency of 2,000,000 Hertz which is greatly attenuated by any metal enclosure. Jeutter describe an epoxy potted receiver which differs from a titanium encased receiver housing.        4) The receiver is capable of automatic switching the supply of power between RF coupled power upon sensing RF energy or battery power upon sensing loss of RF coupled power.        
U.S. Pat. No. 5,279,292 to Joachim Baumann et al. teaches a series resonant circuit in an implantable device. The device disclosed hereinafter uses a parallel resonant circuit which can be tuned at low frequencies, 60 Hz to 500 kHz, such as, for example 8 kHz, which couples well through a titanium enclosure. Further this patent does not teach adjusting charging current as a function of battery temperature or controlling the charging current with the current output from a D/A converter.
U.S. Pat. No. 4,793,353 to William Borkan discloses a non-invasive multiprogramable tissue stimulator which utilizes RF coupling to charge and recharge a capacitor or other rechargeable voltage source. The Borkan circuit differs from the circuit disclosed hereinafter in several respects:                1) First of al in the Borkan circuit, while in the RF stimulation mode, each stimulation pulse must be generated and transmitted by the transmitter, on a real time basis, to the implanted receiver (in contrast, the receiver disclosed hereinafter incorporates all the elements required to autonomously generate and regulate the stimulation pulses);        2) Borkan teaches that a non-rechargeable battery can be used as an alternative power source to RF coupled stimulation, while the circuit disclosed hereinafter uses a rechargeable battery which can be “fast” or “trickle” recharged via low to medium frequency (10 to 500,000 Hertz) RF coupling while Borkan uses a much higher RF coupling (2,000,000 Hertz).        
U.S. Pat. No. 4,408,607 to Donald Maurer teaches a capacitive energy source and associated circuitry for powering a medical apparatus. The Maurer circuitry differs from our the circuitry disclosed hereinafter in that:                1) The Maurer battery is non-rechargeable and only is used to power the implanted receiver during charging of the capacitor (the main power source);        2) Maurer provides no means for non-invasively recharging the battery;        3) Maurer does not control the charging current to a battery relative to the temperature of the battery to prevent gas generation by the battery and loss of battery electrolyte.        
Moreover, Maurer does not teach the capability to switch automatically between RF coupled (power upon detection of RF energy) and battery power (upon sensing absence of RF energy).